Decomposition of complex sulphate salts containing alkali sulphates and alkaline earth sulphates



Patented Nov. 15, 1938 PATENT OFFICE DECOMPOSITION OF COMPLEX SULPHATE SALTS CONTAINING ALKALI SULPHATES AND ALKALINE EARTH SULPHATES Erich Wiedbrauck, Hans Schrader, and Karl Biiche, Essen-Ruhr, Germany, assignors to the firm of Th. Goldschmidt A.-G., Essen-Ruhr,

Germany No Drawing. Application April 13, 1932, Serial No. 605,116. In Germany April 20, 1931 9 Claims.

This invention relates to the decomposition of complex sulphate salts containing alkali sulphates and alkaline earth sulphates, and it comprises a process wherein aqueous liquid containing said complex sulphate salts is superheated to a high temperature and at a high autogenous pressure, the'temperature being usually of the order of the decomposition temperature of the complex salts, thereby giving said alkaline earth l0 sulphates a maximum insolubility and freeing them of combined and adsorbed soluble matter, said process being useful in breaking up various compound or double salts of calcium. sulphate and alkali metal sulphates, such as the double lo 'salts, with potassium sulphate know as syngenite and pentasulphate, and also useful further in the production of formates from alkali metal sulphates with the aid of CO and lime and in the formation of potassium nitrate from potas- 20 sium sulphate with calcium nitrate, etc.; all as more fully hereinafter set forth and as claimed.

During reactions between sulphates of the alkali metals with salts of the alkaline earth metals there are frequently produced rather in- 25 soluble complex sulphatesgbroken up with difiie culty. For example in all reactions in which calcium sulphate is precipitated in the presence of an alkali metal sulphate the calcium sulphate always carries down substantial amounts of alkali 30 metal sulphate in firmly combined form. In all reactions between lime salts and potassium sulphate in solution there are formed double potassium-sulphates; the composition varying with circumstances. Combinations are known of the 35-type CaSO4.KzSO4.aq. (syngenite) and ("pentasulphate). In the reaction of calcium formatewith sodium sulphate a combination of the type NaiCa3(SO4)4(OH)2 has been observed. Some of these multiple salts occur in nature. Much attention has been devoted to economical and elficient ways of working up polyhalite; K2SO4.MgSO4.2CaSO4.2I-I2Q.

All these multiple salts have the common property of holding the alkali metal salt in a. fixedly combined state; audit is therefore difficult toregain the valuable alkali metal sulphates and toquantitatively separate them from the alka- *line earth metal sulphates or even from magnesium sulphate. I i

The question of regaining thealkali metal values from the above described complex salts is an important technological problem and many efforts have been made to solve it in economically satisfactory fashion. Leaching the multiple salts with cold and with hot water and with solutions of various chemicals has been tried. The multiple salts have also been dehydrated as far as possible, heated in a dry state and then treated with hot water at a temperature of around 100 C. In all these methods of the prior art, it has proved impossible to separate alkali metal salts from the alkaline earth metal salts in an economically satisfactory manner. In these methods it is necessary to employ water in too great amounts with too long time of action and the product is an extremely dilute solution from which the salts can be regained only at the expense of high evaporation costs.

It has now been discovered that a satisfactory separation of alkali metal salts from alkaline earth metal salts results when the multiple salts are heated with water or aqueous solutions in a closed autoclave at a decomposition temperature ranging from about 220 to 225 C. This reaction is of quite general utility in the decomposition of complex sulphate salts containing alkali sulphates and alkaline earth sulphates. The necessary temperature depends upon the kind of sulphates contained in the complex sulphate salts.

In the utilization of the reaction, operation can be in several ways. The double salts, as, for instance, those obtained by reacting lime salts with alkali metal sulphates, can be separated in more or less pure form from the reaction mixture and then subjected toa heat and pressure treatment in the presence of suitable solutions. It is however simpler and more economical, after the end "of the reaction not to isolate the reaction products, but to subject the whole reaction mixture to the heat and pressure treatment. This saves the inconvenient and expensive separation of the double sulphates containing alkali sulphates. In many cases, it has proved desirable to conduct the reaction between the lime salts and the alkali metal sulphates in closed vessels with heating to temperatures which are at, or above, the decomposition temperatures of the double sulphates, so that the multiple salts do not form at all. y

The new method is applicable both to the reaction of pure alkali metal sulphates with lime salts and to the direct working up of multiple sulphates containing alkali metal sulphate, both those occurring in nature and those precipitating in various chemical reactions. If the multiple salts contain magnesia as well as alkali metal sulphates and alkaline earth metal sulphates, especially where the magnesium is present as sulreaction. Even slight OH concentrations accel v erate the reaction and render possible decomposition in continuous operation. This is of great advantage in technical embodiments of the reac tion. It is best to use quicklime in securing the alkaline reaction. The time and manner of addition will depend on the operation. It is, however, desirable to have a certain alkalinity of the solution which is to be decomposed.

Additions of lime may be used not only to precipitate magnesia, but also, if desired, to remove some or much of the sulphuric acid of the sulphates. This may be accomplished, for example, 1

by introducing CO as water gas or producer gas into the system under pressure, thereby converting alkali metal sulphate into alkali metal formate with precipitation of calcium sulphate.

The present invention can be conveniently used in Working up syngenite and polyhalite for the contained alkali sulphates. Or the alkali sulphates in these salts can be recovered as potassium nitrate or chlorate with the aid of calcium nitrate or chlorate, for example. In fact any soluble calcium salt can be added to the reaction mixture and the corresponding potassium salt recovered.

The pressure-heat treatment can be effected in any convenient apparatus, but, as a rule, the

multiple salts are brought into reaction with the aqueous solutions in autoclaves provided with a stirrer, heating being for a short time. The alkali metal sulphate goes into solution and reacts according to the nature of the reaction solution. Pure insoluble calcium sulphate is formed, sometimes containing insoluble magnesium salts. Separation of the residue from the solution can be effected by passing the hot reaction mixture,

7 still under pressure, through suitable pressure filters. Or the reaction. mixture can be cooled below its boiling point and then filtered in. the usual manner.

The time required for the reaction varies and .is sometimes rather long, if the solution is not I alkaline. For this reason, the use of an alkaline reacting solution is generally advisable. As stated, with alkaline reactions, the operation is often rapid enough to permit continuous Work; the use of apparatus operating in transit. With the solution alkaline, it is often possible to ope erate at lower temperatures and also at lower pressures without unduly prolonging the heating time.

Calcium sulphate formed by precipitation in solution not only carries down alkali metal sulphates with it, but considerable amounts of other salts in a combined or adsorbed condition, as the case may be thus forming double or multiple salts which are highly insoluble. In using the heat and pressure treatment of the present invention, calcium sulphate can be freed eifectively of adsorbed solubles; not only alkali sulphates but other solubles. In the absence of magnesia, precipitates of calcium sulphate can be obtained which are practically pure and are in. a physical form requiring little washing.

In a practical embodiment of the present invention, wherein potassium nitrate is produced, 200 parts by weight of commercial potassium sulphate and 188 parts by weight of calcium nitrate are brought into reaction with the aid of 350 parts of Water at 80 to 90 C. The mixture is then heated in an autoclave having a stirrer for 3 hours at 250 C. with a gage pressure of about 38 atmospheres; The reaction mixture is cooled to about 90 C. and the calcium sulphate filtered off. The mother liquor is a 38.8 per cent solution of potassium nitrate which will deposit the bulk of its potassium nitrate content on cooling to 20 C. The washed and dried calcium sul- 'phate amounts to 149 parts by weight and carries only 0.3 per cent of the potassium sulphate used. Performing the reaction in the same Way, but without'the pressure-heat treatment, this residue would ordinarily carry 14.5 per cent of the potassium sulphate used. Calcium chlorate can be substituted for the nitrate used in the above example, potassium chlorate being recovered.

In another embodiment of the present invention, wherein potassium nitrate was produced and syngenite used as a source of potash, 245 parts by weight of syngenite were used, this corresponding to 100 parts of potassium sulphate and 103 parts of calcium sulphate. This amount of syngenite was caused to undergo reaction for 3 hours with 94 parts of calcium nitrate in the presence of 174 parts of water, at 250 C. This operation was conducted in an autoclave equipped with a stirrer and at a gage pressure of 38 atmospheres. The reaction mixture was cooled to 150 C. and filtered under atmospheres pressure. A 36.4 per cent solution of potassium nitrate was obtained, the greater part of this salt separating out in pure form from the solution on cooling. The dry residue of calcium sulphate amounted to 181 parts by weight and contained only 2 per cent of the potassium sulphate employed.

In another embodiment, polyhalite was used in the production of potassium nitrate. This polyhalite gave an analysis equivalent to 58.95 per cent calcium sulphate, per cent magnesium sulphate, 16.15 per cent potassium sulphate and 1.32 per cent potassium chloride, the rest being Water. A mixture of 534 parts by weight of polyhalite, 53.4 parts of hydrated lime, 184 parts of a 44.2 per cent calcium nitrate solution and. 185 parts of water, was heated for 3 hours in a closed shaking autoclave at 250 C. and 38 atmospheres. After cooling to 90 C., the mixture was filtered, giving a 23 per cent solution of potash saltpeter. The washed and dried press cake contained only 1.8 per cent of the potash used.

In producing potassium sulphate from polyhalite, 500 parts by weight of polyhalite, 40 parts of quicklime and 800 parts of Water were heated to 220 C. in an autoclave and stirred for 180 minutes. The gage pressure was 23 atmospheres. The mother liquor was separated by filtration under pressure and contained 9.8 per cent K2504. The dried press cake of magnesia and calcium sulphate carried 5 per cent of the K going into reaction.

In producing potassium sulphate from syngenite, 245 parts of syngenite and 1000 parts water were heated to 220-230 C. in an autoclove having a stirrer. The mixture was made slightly alkaline with lime. Heating was for 3 hours and the gage pressure was around atmospheres. After filtration, the mother liquor contained 9.9 per cent K2804 and the press cake carried 1 per cent ofthe K20 of. the syngenite.

Whatwe claim is:

1. In the recovery of alkali metal values from complex salts containing, alkali and alkaline earth metal sulphates, the process which comprises digesting said complex salts with an aqueous medium under super-atmospheric autogenous pressures, at temperatures ranging from about 220 to 250 C. and in the presence of suflicient added lime to render the solution alkaline and to precipitate any magnesium present, separating the precipitate and recovering the alkali metal values from the solution.

2. In the recovery of potassium sulphate from its complex compounds with calcium sulphate, the process which comprises digesting such a complex compound with water, heating the mixture in a closed vessel under super-atmospheric autogenous pressures at a temperature not substantially below 220 C. thereby rendering said potassium sulphate soluble, separating the resulting precipitate and recovering potassium sulphate from the solution, sufi'icient caustic alkali being present to precipitate any magnesium present in said complex compounds.

3. In the recovery of alkali metal values from complex salts consisting substantially of alkali metal and calcium sulphates, the process which comprises digesting said complex salts with an aqueous medium under super-atmospheric autogenous pressures and at elevated temperatures not substantially below 220 C. capable of producing the pressure-heat break down of said complex salts to simple salts, separating the precipitated calcium sulphate and recovering the alkali metal values from the solution.

4. The process of claim 3 wherein the complex salts employed are obtained from the reaction between alkali metal sulphates with calcium salts.

5. The process of claim 3 wherein the complex salts employed are obtained from the reaction between alkali metal sulphates with calcium salts, followed by separation of said complex salts from the reaction mixture.

6. In the process of reacting an alkali metal sulphate with a soluble calcium salt for recovery of the corresponding alkali metal salt, the step which comprises conducting the said reaction under super-atmospheric autogenous pressures and at elevated temperatures not substantially below 220 C. preventing the formation of complex alkali metal-calcium sulphates.

7. In the recovery of alkali metal values from complex salts consisting substantially of alkali metal and calcium sulphates, the process which comprises digesting said complex salts under super-atmospheric autogenous pressure with an aqueous medium having an alkaline reaction at elevated temperatures not substantially below 220 C. capable of producing the pressure-heat break down of said complex salts to simple salts, separating the calcium sulphate thereby precipitated and recovering the alkali metal values from the solution.

8. In the recovery of alkali metal values from complex salts consisting substantially of alkali metal and. calcium sulphates, the process which comprises digesting said complex salts under super-atmospheric autogenous pressure with an aqueous medium in the presence of suflicient added lime to render the solution alkaline, at elevated temperatures not substantially below 220 C., capable of producing the pressure-heat break down of said complex salts to simple salts, separating the calcium sulphate thereby precipitated and recovering the alkali metal values from the solution.

9. In the recovery of alkali metal values from complex salts containing alkali and alkaline earth metal sulphates, the process which comprises digesting said complex salts with an aqueous medium under super-atmospheric autogenous pressures and at elevated temperatures not substantially below 220 C. capable of producing the pressure-heat break down of said complex salts to simple salts, caustic alkali being added to the reaction mixture in an amount sufficient to precipitate any magnesium contained therein, separating the alkaline earth sulphates and recovering the alkali metal values from the solution.

ERICH WIEDBRAUCK. HANS SCHRADER. KARL BiicHE. 

